CS8900A
Crystal LAN™ ISA Ethernet Controller
4.0 PACKETPAGE ARCHITECTURE
4.1 PacketPage Overview
4.1.2 Bus Interface Registers
The Bus Interface registers are used to configure
the CS8900A’s ISA-bus interface and to map the
CS8900A into the host system’s I/O and Memory
space. Most of these registers are written only dur-
ing initialization, remaining unchanged while the
CS8900A is in normal operating mode. The excep-
tions to this are the DMA registers which are mod-
ified continually whenever the CS8900A is using
DMA. These registers are described in more detail
in Section 4.3 on page 42.
The CS8900A architecture is based on a unique,
highly-efficient method of accessing internal regis-
ters and buffer memory known as PacketPage.
PacketPage provides a unified way of controlling
the CS8900A in Memory or I/O space that mini-
mizes CPU overhead and simplifies software. It
provides a flexible set of performance features and
configuration options, allowing designers to devel-
op Ethernet circuits that meet their particular sys-
tem requirements.
4.1.3 Status and Control Registers
The Status and Control registers are the primary
means of controlling and getting status of the
CS8900A. They are described in more detail in
Section 4.4 on page 47.
4.1.1 Integrated Memory
Central to the CS8900A architecture is a 4-Kbyte
page of integrated RAM known as PacketPage
memory. PacketPage memory is used for tempo-
rary storage of transmit and receive frames, and for
internal registers. Access to this memory is done
directly, through Memory space operations
(Section 4.9 on page 74), or indirectly, through I/O
space operations (Section 4.10 on page 76). In
most cases, Memory Mode will provide the best
overall performance, because ISA Memory opera-
tions require fewer cycles than I/O operations. I/O
Mode is the CS8900A’s default configuration and
is used when memory space is not available or
when special operations are required (e.g. waking
the CS8900A from the Software Suspend State re-
quires the host to write to the CS8900A’s assigned
I/O space).
4.1.4 Initiate Transmit Registers
The TxCMD/TxLength registers are used to initiate
Ethernet frame transmission. These registers are
described in more detail in Section 4.5 on page 71.
(See Section 5.7 on page 99 for a description of
frame transmission.)
4.1.5 Address Filter Registers
The Filter registers store the Individual Address fil-
ter and Logical Address filter used by the Destina-
tion Address (DA) filter. These registers are
described in more detail in Section 4.6 on page 72.
For a description of the DA filter, see Section 5.3
on page 87.
The user-accessible portion of PacketPage memory
is organized into the following six sections:
4.1.6 Receive and Transmit Frame Locations
The Receive and Transmit Frame PacketPage loca-
tions are used to transfer Ethernet frames to and
from the host. The host simply writes to and reads
from these locations and internal buffer memory is
dynamically allocated between transmit and re-
ceive as needed. This provides more efficient use
of buffer memory and better overall network per-
formance. As a result of this dynamic allocation,
only one receive frame (starting at PacketPage base
+ 0400h) and one transmit frame (starting at Pack-
PacketPage
Address
Contents
0000h - 0045h
0100h - 013Fh
0140h - 014Fh
0150h - 015Dh
0400h
Bus Interface Registers
Status and Control Registers
Initiate Transmit Registers
Address Filter Registers
Receive Frame Location
Transmit Frame Location
0A00h
CIRRUS LOGIC PRODUCT DATA SHEET
DS271PP3
39
CIRRUS [ CIRRUS LOGIC ]
